Contacting semi-metallic electrical conductors



TEMPERATURE CENT/GRADE Oct. 29, 1957 CONTACTING SEMI-METALLIC ELECTRICAL CONDUCTORS Filed Dec. 15, 1954 R. E. FREDRICK ET AL 2,811,569

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United States Patent CONTACTING SEMI-METALLIC ELECTRICAL CONDUCT ORS Russell E. Fredrick, Milwaukee, Robert W. F1'itts, EI1n Grove, and William V. Huck, Milwaukee, Wis., essignors to Milwaukee Gas Specialty Company, Milwaukee, Wis., a corporation of Wisconsin Application December 15, 1954, Serial No. 475,539 27 Claims. (Cl. 136-5) This application is a continuation-impart of application Serial No. 442,866, now abandoned, filed July 12, 1954, which is in turn a continuation-in-part of Serial No. 366,238, now abandoned, filed July 6, 1953. The invention relates to new articles of manufacture comprising electrical conductors having contact electrodes, the electrical conductors comprising semi-metallic alloys of lead and selenium, lead and tellurium, lead and selenium and tellurium, or any of the foregoing compositions to which promoters have been added, and the contact electrodes being of iron.

Electrical conductors of the aforementioned composi tions, and to which the invention pertains, are disclosed in the copending application of Sebastian Karrer, Serial No. 475,540, filed December 15, 1954, and the -copending application of Robert W. Fritts andISebastian Karrer, Serial No. 475,488, filed December 15, 1954. These electrical conductors afford relatively high thermoelectric power, low resistivity, and low thermal conductivity, have particular utility as thermoelectric elements. A major-obstacle to use of electrical conductors of this character for this and other purposes has been, as we have found, the difficulty of making electrical contact .to the conductors without encountering an alloying or solution of the electrode in the conductors. Such alloying or solution between the electrical conductor and the electrode causes a change in the composition of the electrical conductor which generally results in a reduction-of the high thermoelectric power, hence, such alloying or solution must be controllably restricted if uniformity of the electricalrproperties and long lifetime of the electrical conductor are desired. Thus, for an electrode material which is usable with electrical conductors of the aforementioned compositions, it is necessary that there be no tendency for the electrode and electrical conductors to alloy with or dissolve one another at any temperature within the rangeof operating temperatures of the electrical'eonductor. is a very stringent requirement as the selenium-aud-tellurium constituents of the electrical conductor can readily alloy with or dissolve most of the metals commonly considered to be electrode materials. Moreover, the upper limit of operating temperatures (up to approximately 570 C.) for compositions of which the electrical conductors are made is in the range where extensive alloying=orsolution is common.

The contact electrodes of electrical conductors :of the aforementioned compositions may be either of a pressure type or a bonded type, as hereinafter pointed out. In either case a contact of low thermal and electricaliresistance must be maintained, the electrical resistance being preferably negligible compared to the resistivity of the electrical conductor. Moreover, inthe ease -of the bonded electrode the electrical conductor-electrode interface should have a mechanical strength .atleast comparable to that of the electrical conductor itself.

A primary object of this invention is to provide electrical conductors having contact electrodes in contiguous but which will not alloy with nor dissolve in electrical conductors of'the aforementioned ;pressed in atomic percent.

.are kept within the aforementioned 2,811,569 Patented Oct. 29, 1957 compositions throughout the entire operating temperature range of the unit.

Another object is to provide electrical conductors having contact electrodes of the aforementioned character affording a contact of low thermal and electrical resistance.

Another object is to provide electrical conductors having contact electrodes of the aforementioned character wherein the junction of the electrode and the electrical conductor is mechanically strong.

Another object is to provide methods of making articles of the aforeindicated character comprising electrical conductors of the aforementioned compositions and having contact electrodes, and more particularly, to provide methods of forming bonded connections between the electrical conductor and the contact electrode while nevertheless controllably restricting the alloying or solution .between the electrode and the electrical conductor in order topreserve the electrical properties of the latter.

Other objects and advantages will hereinafter be mentioned or will become apparent to those skilled in the art from the following description and drawings illustrating an embodiment of the invention.

In the drawings:

Figure 'l is a somewhat schematic representation of an embodiment of the invention;

Figure 2 is a graphic illustration of the manner in which iron reduces the melting point of the compositions of which the electrical conductors of the invention are made.

Figure 3 is a graphic illustration of the lead-seleniumtellurium conductors of the invention;

Figure 4 is a liquidus curve for conductors of the afore' mentioned compositions.

In the drawing the reference character 10 designates an electrical conductor which may be made of any of the compositions of the aforementioned copending applications. The electrical conductor 10 may be made of compositions disclosed in the applications above first referred to as, for example, lead and either one or both of the elements selenium and tellurium in which the lead content ranges linearly from a minimum of 61.9% by weight and a maximum of 65.0% by weight, when the balance is substantially all tellurium, to a minimum of 72.45% by weight and a maximum of 75.0% by weight when the balance is substantially all selenium, as graphically illus-- trated in Figure 3, wherein the lead range is plotted against varying proportions of tellurium and selenium, e1-

These compositions should also not contain deleterious metallic impurities of an order in excess of 0.01% and should be oxygen free. It is known that selenium and tellurium are commonly present one in the other and for purposes of this invention suitableelectrical conductor elements may be produced containingboth selenium and tellurium provided the proportions of these constituents with respect to the lead content ranges.

In fact, it is thought to be impossible to achieve absolute .view of which it is to be understood that compositions of .leadantl tellurium in which lead is present in the range of 61.95% to 65.0% by weight may in fact contain so small an amount of selenium as to defy detection but which nevertheless may be assumed to be present due to theimpossibility of achieving absolute purity. Similarly, an alloy or composition of lead and selenium in which lead is present in the range of 72.45% to 75.0% may in fact contain an undetectable amount of tellurium. Compositions within this range afford high thermoelectric power and low resistivity and are alsocharacterized by good mechanical strength and high chemical stability even at elevated temperatures. For example, a composition containing 63.0% lead, balance substantially all tellurium.

after high temperature annealing, has a thermal E. M. F. of minus 275 microvolts per degree C. against copper. (The electrical resistivities of the aforedescribed compositions have positive temperature coefiicients.) A centimeter cube of a lead-tellurium composition containing 62.5% lead, balance substantially all tellurium, delivers about 1.5 watts to a match external load when a temperature difference of 555 C. is maintained between opposite faces and said faces connected to an electric circuit. Furthermore, compositions in this range have compressive strengths greater than 700 kilograms per square centimeter and electrical conductors of compositions in this range utilized as thermocouple elements can be operated with the hot junction at 570 C. in an inert or reducing atmosphere. Electrical conductors exceeding the range of from a minimum of 61.95% to a maximum of 65.0% lead, balance substantially all tellurium, to a minimum of 72.45% and a maximum of 75.0% lead, balance substantially all selenium, will not provide conductors affording all of the desired electrical and physical properties aforementioned.

Compositions of lead, selenium and tellurium affording the desired electrical and physical properties aforementioned may be produced by the following method: The starting constituents, free of metallic contaminants as aforeindicated and preferably in a reduced state are mixed together in the proportions indicated hereinabove and sealed in a tube or container, preferably of quartz or Vycor, the container first being evacuated. The tube and its contents are then heated to the melting point of the latter which occurs at a temperature of from about 920 C. to 1085" C. depending upon the proportions of the selenium and tellurium constituents. This is best illus trated in Figure 4, which is a liquidus curve indicating at what temperature a melt of specific composition begins to freeze. During such heating the molten mass is preferably agitated to insure good mixing, and then cooled.

After the composition has been formed as aforementioned, the solidified ingot can be removed from the tube and cast in molds of graphite or the like under an atmosphere of inert gas. More specifically, during casting we have found it preferable to cover the mold with an inert gas such as, for example, argon or carbon dioxide under a positive pressure. This gas suppresses the vaporization rate of the molten composition thereby reducing porosity of the casting.

The aforedescribed alloying and casting steps should be carried out in crucibles which do not react with or contaminate the composition, since, as aforeindicated, minor amounts of undesired impurity may very deleteriously affect the electrical and/or physical properties of the element. Sutiable crucibles are. we have found, those made of carbon. Alundum, pre-fired lavite and Vycor or quartz.

After casting. the ingots may be machined if necessary. The shaped ingots are then preferably annealed in a reducing atmosphere at from 540 C. to 815 C. for from 20 to hours. This annealing treatment insures homogeneity of the ingot and enhances its electrical and physical properties.

An alternate method of forming the composition comprises melting the constituents as aforementioned in an open crucible under an atmosphere of hydrogen, argon, or any inert and/or reducing gas. Since the vapor pressure of selenium is relatively high and since the formation of HzSe is favored at high temperatures, some loss of selenium may be encountered and adjustments in the initial amount of this constituent must be made to account for this loss. In all other respects this method is similar to that previously described.

The elements of the invention can be achieved through utilization of starting constituents of such purity that the composition of the resultant element contains no more impurity than that aforeindicated. Alternatively, an element of such purity may be afforded by forming the composition, as will hereinafter be described, from impure starting constituents and reducing the impurity content by recrystallization from the melt. As will be understood by those skilled in the art, this comprises melting the impure composition and causing slow solidification thereof progressively from one end of the melt to the other. This results in a concentration of the impurities of the starting constituents in the region of the last point of the melt to solidify, which region may then be discarded and after which the process may, if further purification is necessary, be repeated. Where this method of purification is employed a small amount of additional lead, that is, from approximately 0.1% to 2.5% by weight, must then be added to the resultant composition to adjust the same to bring it within the limits aforeindicated.

Electrical conductors produced in accordance with the present invention exhibit the desired electrical and physical properties aforementioned. Because of the nature of residual impurities which are difficult to remove the electrical properties of a given composition may vary slightly from batch to batch even though the impurity concentration is limited as aforeindicated and the composition content is held within the minimum and maximum ranges shown in Figure 3. Within such limits, however, the electrical properties are reproducible within about plus or minus 10 percent of the values for the electrical characteristics aforementioned by way of example.

The thermal voltage afforded by conductors of the compositions aforedescribed, we have found, is governed primarily by the seleniurn'tellurium proportion in the selenium-tellurium constituent of the composition and is relatively independent of the amount of lead, providing the proportions of lead and selenium-tellurium constituents lie within the limits specified and illustrated in Figure 3.

Compositions of the present invention also exhibit the desired physical properties aforementioned. More specifically, they are mechanically strong and stable under operating conditions. The compositions in the mid-range are more brittle and are more resistant to plastic deformation than are compositions at either end. The coefficients of thermal expansion (linear) lie within the range of from l8 10 C. to 16 l0 C. Moreover, we have found that compositions of the present invention afford higher thermal efficiencies than do metals because of the low thermal conductivity of said compositions (about .02 to .03 w./cm./ C.).

Electrical conductors of the present invention may further comprise the semi-metallic alloys or compositions heretofore discussed which have been promoted by the addition of certain third element additions or third element-promoters. For convenience such compositions will be described by way of example with reference to the compositions graphically illustrated in Figure 3 at the left and righthand extremes respectively, such compositions being generally denominated as terminal compositions. It will be understood, of course, that in essence, any of the compositions illustrated in Figure 3 intermediate such terminal compositions are in effect mixtures thereof and that such intermediate compositions may also be promoted by the addition of third elements, which will then be selected in kind and amount, determined by the proportions of the terminal compositions in such intermediate composition.

The magnitudes of the thermoelectric power and electrical resistivity of the aforementioned lead-tellurium alloys are strongly dependent upon the heat treatment afforded the alloy during fabrication. For example, a lead-tellurium alloy within the above stated range of composition, which has been annealed for several hours at from 540 C. to 815 C. and then quenched, exhibits lower thermoelectric power and electrical resistivity than do the same alloys which have been similarly annealed and then cooled slowly to lower temperatures. The following table, identified as Table I, sets forth representative electrical properties, at room temperatures, of the last noted lead-tellurium alloys as a function of the quench- Tab te I Equilibrium Temperature Prior Thermoelectric Resistivity,

to Quenching Power, Micro- Ohm-Cm.

volts/ C.

316 0. 0023 -3t' 2 0. 0034 -396 0. 0074 -414 l). 0150 414 0. 0173 414 O. 0175 The aforementioned lead-tellurium alloys may be best described metallographically as a two-phase alloy. It has been observed that these alloys, when sectioned and examined microscopically, comprise a maior phase comprising crystal grains varying usually from 1 to millimeters in size and between such grains there exist thin relatively darker regions of a second phase. The grains of the primary phase are crystals of the intermetallic compound lead-telluride which contain approximately 61.89% lead by weight. The darker second phase, clearly discernible at the grain boundaries, is lead containing a minor concentration of tellurium.

The function of the second phase in such alloys is thought to be threefold. First, the thermal equilibrium between the two phases, which is established by the heat treatment aforementioned, induces negative thermoelectric power and conductivity in the primary lead-telluride phase which, because of its high concentration in the alloy, controls the electrical properties of the two-phase alloy. Secondly, the thin layers act as a cementing agent for the grains of the primary phase, thereby improving the mechanical strength of the alloy when compared to that of the pure intermetallic compound. This cementing action of the second phase improves the strength of an alloy in tension and compression at all temperatures up to 815 C. Thirdly, the second phase affords good electrical continuity in the polycrystalline alloy by rendering the intergranular component of electrical resistivity negligible. We have found that the actual concentration of second phase is not critical so long as the composition is maintained within the aforementioned specified ranges.

Lead-tellurium alloys containing less than 61.95% lead by weight do not usually exhibit reproducible physical and electrical properties when in a polycrystalline state, and in alloys containing more than 65.0% lead by weight the second phase regions are of such dimension that the electrical conductivity along the grain boundaries of the alloy cannot be neglected when compared with the conductivity through the primary phase. Moreover, unless the specified ranges of compositions and purity hereinafter described are adhered to, third element addition promoters hereinafter described will be rendered ineffective. Additionally, alloys having large concentrations of the aforementioned second phase are subject to plastic flow at high temperatures and for this further reason are not desirable for high temperature applications. Accordingly, the specified composition ranges and purity are to be considered critical.

It will be observed that since the electrical properties of the aforementioned lead-tellurium base alloy or composition are dependent upon the equilibrium temperature from which they have been quenched, use of such alloys or compositions is limited to such temperatures as will not affect the electrical characteristics established by the quenching treatment. Accordingly, for high temperature applications requiring fixed values of electrical characteristics arbitrary changes in these characteristics must be derived from the adjustment of factors other than temperature and annealing history.

As related in the copending application of Robert W. Fritts and Sebastian Karrer, Serial No. 475,488, filed December 15, 1954, the electrical characteristics of leadtellurium alloys or compositions of the aforedescribed character can be markedly and advantageously altered in a reproducible manner by the addition thereto of controlled amounts of matter other than lead or tellurium. For convenience, these additions are herein designated third element additions or third element-promoters" to distinguish them from the lead and tellurium constituents of the alloys of the invention.

Compositions including third element promoted alloys corresponding to the compositions illustrated in Figure 3 at the lefthand extreme are more fully disclosed in the last referred to copending application of Robert W. Fritts and Sebastian Karrer. More specifically, such compositions may comprise a lead-tellurium base composition or alloy consisting essentially of lead in the range of 61.95% to 63.0% by weight, balance substantially all tellurium, and which base composition contains not more than 0.001% by weight of other matter. Lead-tellurium base alloys within the aforementioned range and of the aforementioned purity are negative electrical conductors and exhibit high negative thermoelectric power, nominally higher electrical resistivity, and low thermal conductivity with respect to a metal. Such compositions or alloys have utility as electrical conductors of the present invention.

The "third element additions" or third element-promoters, effective for electrical conductors of the present invention when added in minor amounts to the leadtellurium base alloy aforementioned are: Bismuth, tantalum, manganese, zirconium, titanium, aluminum, gallium, chlorine, bromine, iodine, uranium, sodium, potassium, thallium and arsenic.

The third element additions or third element-promoters aforementioned may be either positive promoters" or negative promoters as hereinafter defined, and the resultant alloy or composition may be a positive or negative alloy or composition or conductor, as also hereinafter defined.

A negative composition or alloy and a negative conductor is to be understood through this specification as meaning an alloy, composition or conductor which exhibits negative conductivity as evidenced by Hall effect measurements or thermoelectric effect measurements, both taken at room temperature. Similarly, a positive" composition or alloy and a positive" conductor is to be understood as meaning an alloy, composition or conductor which exhibits positive conductivity as evidenced by Hall effect measurements or thermoelectric effect measurements, both taken at room temperature.

Negative promoters" are those which, when added to the lead-tellurium base alloy previously defined, alter the electrical conductivity Without changing the polarity of the conductivity or thermoelectric power of the base alloy (it being negative according to the preceding definition). Positive promoters are those which, when added to the lead-tellurium base alloy, cause at first, with very small additions, reduction in the conductivity of the alloy to a minimum value beyond which further increase in the concentration of the positive promoter causes an increase in the conductivity of the alloy accompanied by a reversal in the polarity of the conductivity and thermoelectric power, i. e., from negative to positive.

The functions of such negative and positive promoters should be contrasted for sake of clarity as follows:

(1) Increasing concentrations of the negative promoter elements cause increases in the conductivity and decrease of the thermoelectric power of the resulting alloy, as compared to that of the leaddellurium base alloy, while preserving the negative polarity of the conductivity and thermoelectric power thereof.

(2) Increasing concentrations of positive promoter elements cause initially reductions in the conductivity and increase in the thermoelectric power of the lead-tellurium base alloy, until a minimum conductivity is reached whereupon the thermoelectric power and conductivity reverse polarity to the positive sense, and further increase in the concentrations of the positive promoter causes increase in the conductivity and decrease of the thermoelectric power in the resulting alloy.

Table II below, first column thereof, lists certain elements which are effective as negative promoters when added to the aforementioned lead-tellurium base alloys or compositions. Second column of Table 11 lists the order of the maximum concentration limits by weight percent of such promoters to the base alloy. It is to be understood that these concentration limits are the maximum which effectively alter the electrical properties of the base alloy. Concentrations in excess of the stated amounts of such additives have no appreciable effect in beneficially altering the electrical properties of the electrical conductors with which this invention is concerned. and in this sense the limits indicated are to be considered critical. The third and fourth columns of Table 11 set forth the electrical properties at room temperature of lead tellurium alloys promoted with the maximum useful concentrations of the negative promoters, after high temperature annealing as hereinafter disclosed.

Table II Order of Maximum Effective Thermoelec- Negatlve Promoters Concentration trio Power, Resistivity,

Limits By Microvoltsl Ohm-Cm.

Weight C. Percent 1 60-1. T 0. 00031 0. 50 121 0. 00032 Manganese. 0. 113 (I. 00036 Zirconinnn. (J. 25 23 0. cont? Titanium... 0. 15 45 0. 00020 Alumtnum. 0. 10 -59 0. 00cm Gallium. 0. 25 0. 00015 chlorinehn 0. 10 0. 00019 Bromine 0.20 47 0.00015 1001118.... (1.25 45 0 00015 Uranium. 0v S0 72 0 00020 1 The. range set forth is dlsrusscd below.

As previously mentioned, certain positive promoters may also be alloyed with the aforementioned lead-tellurium alloys, and such promoters are listed in column 1 of Table III below. The second column of Table ill, like the corresponding column of Table 11, sets forth the order of the maximum concentration limits by weight percent of such promoters to the base alloy effective for providing electrical conductors of the invention. Again, it will be observed that concentrations of the positive promoters to the lead-tellurium base alloy in amounts in excess of that contained in column 2 of Table Ill have no appreciable eti'ect in beneficially altering the electrical properties of the electrical conductors with which this invention is concerned and in this sense the limits indicated are to be considered critical.

Column 3 of Table Ill sets forth the concentration by weight percent of the positive promoters listed at which the polarity of conductivity and thermoelectric power of the promoted alloy reverses.

Columns 4 and 5 set forth the thermoelectric power and resistivity characteristics at room temperature of the alloy or composition resulting from the addition of the aforedescribed positive promoters in the amount shown in column 2 after high temperature annealing and subsequent slow cooling as hereinafter disclosed.

1 The range set forth is discussed below.

As aforementioned, the lead-tellurium base alloy previously described, is a two-phase alloy. When the aforedescribed third element additions are incorporated in the base alloy, such third element additions become distributed between the two phases. We have discovered that the nature of such distribution has negligible effect upon the electrical properties of the composition in all cases except that of bismuth, thallium and arsenic. Accordingly, in the case of bismuth, thallium and arsenic, the maximum effective concentration is dependent upon the lead content of the lead-tellurium base alloy within the ranges stated therefor in Tables II and III. We have found 1.20% by weight bismuth to be the maximum effective concentration for lead-telluriurn base alloys containing 63.0% lead; for base alloys containing less lead the maximum effective bismuth concentration is somewhat less, that is ranges down to 0.60% by weight when the lead content ranges down to 61.95%. Similarly, in the case of thallium, the maximum effective concentration is dependent upon the lead content of the lead-tellurium base alloy within the range stated therefor. We have found 1.00% by weight thallium to be the maximum effective concentration for lead-tellurium base alloys containing 63.0% lead; for base alloys containing less lead, the maximum effective thallium concentration is somewhat less, that is ranges down to .25% by weight when the lead content ranges down to 61.95%. Similarly, in the case of arsenic, the maximum effective concentration is dependent upon the lead content of the lead-tellurium base alloy within the range stated therefor, and ranges from 0.25% for base alloys containing 63.0% lead down to 0.07% for base alloys containing 61.95% lead. As indicated in Table III, the concentration weight percent at which the polarity reverses in the case of thallium promoted base alloy ranges from .005 to .02 as the lead constituent of the lead-tellurium base composition varies from 61.95% to 63.0%. Similarly, in the case of the arsenic promoted base alloy the concentration weight percent at which polarity reverses ranges from .0008 to .002 as the lead content of the base alloy varies from 61.95% to 63.0%. This behavior of bismuth, thallium and arsenic is thought to be due to the formation of a bismuth-lead-tellurium, a thallium-lead-tellurium or an arsenic-leadtellurium complex within the intergranular phase aforementioned which accounts for a portion of the addition. All other third element additions aforementioned, both positive and negative, form complexes with the second or intergranular phase aforementioned to a much lesser extent than do bismuth, thallium and arsenic and for purposes of this invention, in the cases of such other additions these effects are inconsequential. Ac-

' cordingly, no changes in the concentration limits thereof are necessary as the proportions of lead and tellurium in the base alloy vary within the range stated therefor.

In Tables II and III above, the thermoelectric power and resistivity data given is in both cases for the 61.95% lead, balance substantially all tellurium composition containing the third element addition in question in the amount indicated in the table (in the case of bismuth, thallium and arsenic the lower maximum effective amount indicated).

It will be observed upon examination of the data recorded in Tables II and III that a wide range of electrical properties can be induced in lead-tellurium base alloys by third element additions, either positive or negative as desired. Zirconium additions, for example, can reduce the resistivity of the lead-tcllurium base alloy by more than a factor of approximately 100, while reducing the thermoelectric power by a factor of 20.

The aforedescribed lead-tellurium base alloys or compositions and third element promoted alloys and electrical conductors comprising the invention may be fabricated by melting together the alloy constituents aforementioned, within the concentration limits aforeindicated. It is to be understood, however, that, as has been previously indicated, the lead-tellurium alloys of the invention must be of a high order of purity, i. e., containing not in excess of the order of 0.001% by weight impurity. Such purity has been found to be necessary if the electrical properties of the electrical conductors of this invention are to be reproducible. It is to be understood, however, that selenium, as before related, because of its chemical similarity to and natural occurrence with tellurium, is frequently present in commercial tellurium, and is difficult and expensive to remove to the extent of purity as specified above. However, selenium concentrations of the order usually found in commercially pure tellurium, usually of the order of 1%, cause no significant changes in the electrical properties of electrical conductors here under discussion.

In the production of the electrical conductors comprising the alloys or compositions last discussed, the constituents are melted at from at least 920 C. to at least 1100" C. under a reducing atmosphere, and agitated to insure uniform distribution. The alloy may then be cast, formed or machined as desired. It is then preferably annealed to insure normalization of the alloy or composition. Such annealing may be accomplished at temperatures ranging from 540 C. to 815 C. for from to 20 hours, the lesser time being required at the higher temperature. The aforementioned annealing may be conveniently accomplished by sealing the ingots of the alloy or composition within a quartz or Vycor envelope under a hydrogen atmosphere. This prevents loss of material and hence preservation of the ingots during annealing and affords a simple method of handling. After heating, the sealed tube may, in the case of the base leadtellurium alloy, be cooled slowly to the desired quenching temperature depending upon the electrical characteristics desired (see Table I). The third element addition promoted compositions may be slowly cooled to room temperature or quenched directly or at any intermediate temperature in cold water without substantially affecting the reproducibility of the desired electrical characteristics, especially as the concentration of the third element addition approaches its maximum elfective concentration limit aforeindicated. Thus, the heat treatment history of the alloy or composition becomes of lesser importance in the third element addition promoted compositions aforedescribed as the concentration of the third element addition approaches the maximum effective limit aforeindicated.

The third element promoted alley or composition is a two-phase alloy having improved electrical properties as compared to the corresponding properties of the leadtellurium base alloy. For example, the electrical properties of electrical conductors of the third element addition promoted alloys or compositions are governed to a lesser extent by the heat treatment given the alloy, with variations in electrical properties considerably less than the variations exhibited by the lead-tellurium base alloy to which no third element has been added. Thus, the third element additions or third element promoters, in effect, reduce the dependency of the electrical properties upon prior heat treatment and in this sense tend to stabilize these properties to a higher degree than that achieved in the lead-tellurium base alloy. It may be stated as a general observation that the degree of stabilization increases with the concentration of the aforementioned third element additions up to the maximum effective amount thereof as above set forth. This lesser dependency of third element addition promoted alloys or compositions aforedescribed and of electrical conductors comprising the same, markedly increases the utility thereof for high temperature appli cations. In this connection, however, where alloys including positive promoters are concerned and where the application temperature approaches 570 C. concentrations of the positive promoter approaching the maximum effective limit aforementioned should be used to insure maintenance of positive polarity of the composition.

The promoted lead-tellurium compositions aforedescribed also exhibit the desired physical properties aforementioned. More specifically, they are mechanically strong and stable under operating conditions. The coefficient of thermal expansion is of the order of Moreover, we have found that compositions of the C. Moreover, we have found that compositions of the present invention afford higher thermal efficiencies than do metals when utilized as thermoelectric elements because of the low thermal conductivity of said compositions (about .02 w./cm./ Q).

As is the case with the lcad-tellurium alloys aforedescribed, the magnitudes of the thermoelectric power and electrical resistivity of the aforementioned leadselenium alloys are strongly dependent upon the heat treatment afforded the alloy during fabrication. For example, a lead-selenium alloy within the above-stated range of composition, which has been annealed for several hours at from 540 C. to 815 C. and then quenched, exhibits lower thermoelectric power and electrical resistivity than do the same alloys which have been similarly annealed and then cooled slowly to lower temperatures. The following table, identified as Table IV, sets forth representative electrical properties, at room temperature, of the last noted leadselenium alloys as a function of the quenching temperature. The data of Table IV represents leadselenium alloys which have been annealed at from 540 C. to 815 C. and slowly cooled (e. g. 50 C. per hour) to the indicated temperatures in column 1, and from which temperatures they were quenched.

Table IV Equilibrium Temperature Prior to 'lhcrmoclcc- Resistivity, Quenching tric Power, Ohm-Cm.

Microvolts/O.

The aforementioned lead-selenium alloys may be best described metallographically as a two-phase alloy. It has been observed that these alloys, when sectioned and examined microscopically, comprise a major phase comprising crystal grains varying usually from 1 to 10 millimeters in size and between such grains there exist thin relatively darker regions of a second phase. The grains of the primary phase are crystals of the intermetailic compound lead-selenide which contain approximately 72.41% lead by weight. The darker second phase, clearly discernible at the grain boundaries, is lead containing a minor concentration of selenium.

The function of the second phase in such alloys is thought to be threefold. First, the thermal equilibrium between the two phases, which is established by the heat treatment aforementioned, induces negative thermoelectric power and conductivity in the primary lead-selenidc' phase which, because of its high concentration in the alloy, controls the electrical properties of the two-phase alloy. Secondly, the thin layers act as a cementing agent for the grains of the primary phase, thereby improving the mechanical strength of the alloy when compared to that of the pure intermetallic compound. This cementing action of the second phase improves the strength of an alloy in tension and compression at all temperatures up to 815 C. Thirdly, the second phase affords good electrical continuity in the polycrystalline alloy by rendering the intergranuiar component of electrical resistivity negligible. We have found that the actual concentration of second phase is not critical so long as the composition is maintained within the aforementioned specified ranges.

Lead-selenium alloys containing less than 72.45% lead by Weight do not usually exhibit reproducible physical and electrical properties when in a polycrystalline state, and in alloys containing more than 75.0% lead by weight the second phase regions are of such dimension that the electrical conductivity along the grain boundaries of the alloy cannot be neglected when compared with the conductivity through the primary phase. Moreover, unless the specified ranges of compositions and purity hereinafter described are adhered to, third element addition promoters hereinafter described will be rendered ineifective. Additionally, alloys having large concentrations of the aforementioned second phase are subject to plastic flow at high temperatures and for this further reason are not desirable for high temperature applications. Accordingly, the specified composition ranges and purity are to be considered critical.

It will be observed that since the electrical properties of the aforementioned lead-selenium base alloy or composition are dependent upon the equilibrium temperature from which they have been quenched, use of such alloys or compositions is limited to such temperatures as will not affect the electrical characteristics established by the quenching treatment. Accordingly, for high temperature applications requiring fixed values of electrical characteristics arbitrary changes in these characteristics must be derived from the adjustment of factors other than temperature and annealing history.

As further related in the above mentioned copending application of Robert W. Fritts and Sebastian Karrer, Serial No. 475,488, filed December 15, 1954, electrical characteristics of such lead-selenium alloys or compositions can be markedly and advantageously altered in a reproducible manner by the addition thereto of controlled amounts of matter other than lead and selenium. These additions are herein designated third element additions or third element promoters and may be negative or positive promoters all as heretofore defined.

Compositions including third element promoted alloys corresponding to the compositions illustrated in Figure 3 at the right-hand extreme are more fully disclosed in the last-referred to copending application of Robert W. Fritts and Sebastian Karrer, More specifically, such compositions may comprise a lead-selenium base com- 8 position or alloy consisting essentially of lead in the range of 72.45% to 73.50% by weight, balance substantially all selenium, and the base composition containing not more than 0.001% by weight of other matter. Leadseienium base alloys within the aforementioned range and of the aforementioned purity are negative electrical conductors and exhibit high negative thermoelectric power, nominally higher resistivity, and low thermal conductivity with respect to a metal. Such compositions or alloys have utility as electrical conductors of the present invention.

The third element additions which we have found effective for the purposes of the present invention when added in minor amounts to the lead-selenium base alloy aforementioned are: Iodine, chlorine, bromine, zirconium, silicon, titanium, indium, tantalum, gallium, aluminum, copper, gold, bismuth, antimony, fluorine, columbium, sodium, thallium, potassium, lithium and arsenic.

Table V below, first column thereof lists certain elements which we have found effective as negative promoters when added to the aforementioned lead-selenium base alloys or compositions. Second column of Table V lists the order of the maximum concentration limits by weight percent of such promoters to the base alloy effective for achieving the objects of the invention. It is to be understood that these concentration limits are the maximum which eifectively alter the electrical proper ties of the base alloy. Concentrations in excess of the stated amounts of such additives have no appreciable effect in beneficially altering the electrical properties with which this invention is concerned, and in this sense the limits indicated are to be considered critical. The third and fourth columns of Table V set forth the electrical properties at room temperature of lead-selenium alloys promoted with the maximum useful concentrations of the negative promoters, after high temperature annealing as hereinafter disclosed.

Table V Order of Maxi- 'iherinoelcc- Resistivity, Negative Promoters mum Eflcctivc trie Power, Ohm-Cm.

Concentration Microvolts/C.

Limits By Weight Percent Iodine 0. 60 40 0. 00016 Chlorine 0. 20 38 0. 00018 Bromine... 0. 34 0. 00014 giroonium 08 g;

ilicon. 1 Titaniunr. i 0. 10 40 0 00026 Indiurn j 0. 20 -54 0. 00024 Tantalum. 0. 00 45 01 00022 Galliumfl". 0.15 -58 0.00027 Aluminum 0. 03 101 0. 00037 Copper. 0.30 104 0. 00038 Gold. 0. 35 104 0. 00039 Bismuth 1 0. 40-2. 5 0. 00040 Antimony l 0. 20-1. 5 103 0. 00050 iuorine 0. 02 133 0. 00047 Colnrnbium. 0. 35 65 0.00023 l The range set forth is discussed below.

As previously mentioned, certain positive promoters may also be alloyed with the aforementioned lead-selenium alloys, and such promoters are listed in column 1 of Table VI below. The second column of Table VI, like the corresponding column of Table V, sets forth the order of the maximum concentration limits by weight percent of such promoters to the base alloy effective for achieving the objects of the invention. Again, it will be observed that concentrations of the positive promoters to the lead-selenium base alloy in amounts in excess of that contained in column 2 of Table VI have no appreciable effect in beneficially altering the electrical properties with which this invention is concerned and in this sense the limits indicated are to be considered critical.

Column 3 of Table VI sets forth the concentration by weight percent of the positive promoters listed at which the polarity of conductivity and thermoelectric power of the promoted alloy reverses.

Columns 4 and 5 set forth the thermoelectric power and resistivity characteristics at room temperature of the alloy or composition resulting from the addition of the aforedescribed positive promoters in the amount shown in column 2 after high temperature annealing and subsequent slow cooling as hereinafter disclosed.

Table VI Order of Maxi- Concentration, Thcrmo- Positive Promum Effective Weight Percent electric Resistivity,

moters Concentration at Which Po- Power, Ohm-Cm. Limit By larity Reverses Micro- Weight Percent vo1ts/ 0.

Sodium 0. 08 002 0. 0019 Thailitun 1 0. 72-1. 5 1 04-. 08 +263 0. 0108 Potassiurm 0. i5 003 +250 0. 0076 Lithium. 0. 03 .002 +288 0. 0108 Arsenic .1 1 0.100.30 1 .02. 06 0. 0110 1 The range set forth is discussed below.

As aforementioned, the lead-selenium base alloy previously described is a two-phase alloy. When the aforedescribed third element additions are incorporated in the base alloy, such third element additions become distributed between the two phases. We have discovered that the nature of such distribution has negligible effect upon the electrical properties of the composition in all cases except that of bismuth, antimony, thallium and arsenic. Accordingly, in the case of bismuth, antimony, thallium and arsenic, the maximum etfective concentration is dependent upon the lead content of the lead-selenium base alloy within the ranges stated therefor in Tables V and VI. We have found 2.50% by weight bismuth to be the maximum efiective concentration for lead-seleni- .um base alloys containing 73.50% lead; for base alloys containing less lead the maximum effective bismuth concentration is somewhat less, that is ranges down to 0.40% by weight when the lead content ranges down to 72.45%. Similarly, in the case of antimony, the maximum effective concentration is dependent upon the lead content of the lead-selenium base alloy within the range stated therefor. We have found 1.5% by weight antimony to be the maximum efiective concentration for lead-selenium base alloys containing 73.50% lead; for base alloys containing less lead the maximum effective antimony concentration is somewhat less, that is ranges down to 0.20% by weight when the lead content ranges down to 72.45%. Similarly, in the case of thallium, the maximum effective concentration is dependent upon the lead content of the leadselenium base alloy within the range stated therefor. We have found 1.5% by weight thallium to be the maximum efiective concentration for lead-selenium base alloys containing 73.50% lead; for base alloys containing less lead, the maximum effective thallium concentration is somewhat less, that is ranges down to .72% by weight when the lead content ranges down to 72.45%. Similarly, in the case of arsenic, the maximum effective concentration is dependent upon the lead content of the lead-selenium base alloy within the range stated therefor, and ranges from 0.30% for base alloys containing 73.50% lead down to 0.10% for 'base alloys containing 72.45% lead. As indicated in Table VI, the concentration weight percent at which the polarity reverses in the case of the thallium pro- .moted base alloy ranges from 0.04% to 0.08% as the lead constituent of the lead-selenium base composition varies from 72.45% to 73.50%. Similarly, in the case .of the arsenic promoted base alloy the concentration weight percent at which polarity reverses ranges from 0.02% to 0.06% as the lead content of the base alloy varies from 72.45% to 73.50%. This behavior of hismouth, antimony, thallium and arsenic is thought to be due to the formation of a bismuth-lead-selenium, an anttmony-lead-selenium, a thallium-lead-selenium or an arsenicdead selenium complex within the intergranular phase aforementioned which accounts for a portion of the addition. All other third element additions or third element promoters aforementioned, both positive and negative, form complexes with the second or intergranular phase aforementioned to a much lesser extent than do bismuth, antimony, thallium and arsenic and for purposes of this invention, in the cases of such other additions these effects are inconsequential. Accordingly, no changes in the concentration limits thereof are necessary as the proportions of lead and selenium in the base alloy vary within the range stated therefor.

In Tables V and VI above, the thermoelectric power and resistivity data given is in both cases for the 72.45% lead, balance substantially all selenium composition containing the third element addition in question in the amount indicated in the table (in the case of bismuth, antimony, thallium and arsenic, the lower maximum effective amount indicated).

It will be observed upon recorded in Figures 1 through 6,

examination of the data that a wide range of electrical properties can be induced in lead-selenium base alloys by third element additions or third element promoters, either positive or negative as desired. Bromine additions, for example, can reduce the resistivity of the lead-selenium base alloy by more than a factor of approximately 20, while reducing the thermoelectric power by a factor of 10.

The aforedescribed alloys or compositions and electrical conductors comprising the invention may be fabricated by melting together the alloy constituents aforementioned, within the concentration limits aforeindicated. It is to be understood, however, that as has been previously indicated, the lead-selenium alloys of the invention must be of a high order of purity, i. e., containing not in excess of the order of 0.001% by weight impurity. Such purity has been found to be necessary in practicing the present invention if the electrical properties of the alloys of this invention are to be reproducible. It is to be understood, however, that telluriurn and sulphur, because of their chemical similarity to and natural occurrence with selenium, are frequently contaminants in commercial selenium and are difiicult and expensive to remove to the extent of purity as specified above. We have found, however, that tellurium and sulphur concentrations of the order usually found in commercially pure selenium, usually of the order of 1%, cause no significant changes in the electrical properties of the alloys of this invention.

In the production of the new alloys or compositions of our invention and electrical conductors comprising the same, the constituents are melted at from 1085 C. to 1200 C. under a reducing atmosphere, and agitated to insure uniform distribution. The alloy may then be cast, formed or machined as desired. lt is then preferably annealed to insure normalization of the alloy or composition. Such annealing may be accomplished at temperatures ranging from 540 C. to 8l5 C. for from 10 to 20 hours, the lesser time being required at the higher temperature. The aforementioned annealing may be conveniently accomplished by sealing the ingots of the alloy or composition within a quartz or Vycor envelope under a hydrogen atmosphere. This prevents loss of material and hence preservation of the ingots during annealing and affords a simple method of handling. The aforedescribed third element addition promoted compositions may then be slowly cooled to room temperature or quenched directly or at any intermediate temperature in cold water without substantially affecting the reproducibility of the desired electrical Characteristics, especially as the concentration of the third element addition approaches its maximum effective concentration limit aforeindicated. Thus, the heat treatment history of the alloy or composition becomes of lesser importance in the third element addition promoted compositions aforedescribed as the concentration of the third element addition approaches the maximum effective limit aforeindicated.

The third element promoted alloy or composition is a two-phase alloy having improved electrical properties as compared to the corresponding properties of the leadselenium base alloy. For example, the electrical properties of the third element addition promoted alloys or compositions are governed to a lesser extent by the heat treatment given the alloy, with variations in electrical properties considerably less than the variations exhibited by the lead-selenium base alloy to which no third element has been added. Thus, the third element additions or third element-promoters, in effect, reduce the dependency of the electrical properties upon prior heat treatment and in this sense tend to stabilize these properties to a higher degree than that achieved in the lead-selenium base alloy. It may he stated as a general observation that the degree of stabilization increases with the concentration of the aforementioned third element additions or third element-promoters up to the maximum effective amount thereof as above set forth. This lesser dependency of third element addition promoted alloys or compositions aforedescribed and of electrical conductors comprising the same, markedly increases the utility thereof for high temperature applications. In this connection, however, where alloys including positive promoters are concerned and where the application temperature ap proaches 570 C. concentrations of the positive promoter approaching the maximum effective limit aforementioned should be used to insure maintenance of positive polarity of the composition.

The promoted lead-selenium compositions aforedescribed also exhibit the desired physical properties aforementioned. More specifically, they are mechanically strong and stable under operating conditions. The coefficient of thermal expansion is of the order of l6 10 C. Moreover, we have found that compositions of the present invention afford higher thermal efiiciences than do metals when utilized as thermoelectric generator elements because of the low thermal conductivity of said compositions (about .03 w./cm./

Contact electrodes designated at 11 and 12, respectively, are illustrated in Figure 1 as being applied to opposite ends of an element of any of the aforesaid lead-selenium, lead-tellurium, lead-tellurium-selenium, and third element addition promoted compositions, it being understood that one or more electrodes may be utilized, and that such electrodes may be in contact with the element 10 at any desired point thereon. The interfaces of the contact electrode 11 and 12 with the element 10 are indicated at A and A, respectively, which interfaces are to be understood to be microscopically irregular and, in fact, interlocking, for a bonded electrode, as will hereinafter appear.

We have discovered that for under consideration, a contact electrode comprising iron or certain iron alloys affords the desired low electrical contact resistance, chemical stability, and for bonded elec trodes, mechanically strong bonds and low thermal contact resistance.

Iron is acceptable as an electrode for conductors of the various compositions aforedescribed since, we have discovered, it does not alloy or dissolve in the conductor at temperatures below 700 C., which is well above the ordinary upper limit conductor. We have further discovered that an alloying or solution between the conductor and iron or iron alloy contact electrode takes place at above 730 C., thereby permitting bonded contacts to be formed very simply, as will hereinafter appear. Moreover, a reversal of the altoying or solution below approximately 700 C.,

elements of the character the amount of such dispersed iron is controlled, as Will hereinafter be described, but also the presence of such minor iron concentration, we have found, increases the It is, however, important that the iron be so cont olled in amount and dispersed so that there will be no serious effect upon the thermoelectric properties of the conductor. found that ifthe iron concentration is held to less than A method of forming bonded contact electrodes of iron of operating temperatures of the L all) with conductors of the aforedescribed compositions stems from our discovery that iron dissolves slowly in such compositions within the range 715 C. to 730 C. (the former temperature representing that end of the previously described composition range of the composition wherein tellurium is but a trace, and the latter temperature being that end of the previously described composition range of the conductor wherein selenium is but a trace in composition), that the conductors of the various compositions aforedescribed exhibit reduced melting points when laden with a few percent iron, and that, in fact, such melting points may lie below the phase transformation temperature of iron (905 Q). Where the con ductor is of composition lying at the end of the range where selenium is but a trace, as little as 2.0% by weight iron affords an alloy having a melting point below the aforementioned transformation temperature of pure iron. Similarly, where the composition of the conductor is at the other end of the range. that is, contains but a trace of tellurium, as little as 9.0% by weight iron has a similar effect. This is graphically illustrated in Figure 2 wherein curve B illustrates the lowering of the melting point of a conductor of the afcredescribed composition containing but a trace of selenium with iron additions, and curve C similarly illustrate; the lowering of melting point of a conductor of the aforedescribed composition containing but a trace of tellurium with additions of iron. (Lead-tellurium compositions promoted by the addition of third element additions aforedescribed are also illustrated by curve B and lead-selenium compositions promoted by the addition of third element additions are also illustrated by curve C.) As will be apparent, this discovery provides a simpie technique by which bonded electrodes may be formed with iron or iron alloys since contact formation can take place at a temperature below that at which the phase transformtion of iron occurs. Moreover, the method utilizing this discovery, about to be described, results in considerably less contamination of the conductor with iron after contacting than the .5% limit aforementioned. In fact, We have found the contamination resulting from this method generally to be less than a few hundredths of one percent.

The method utilizing iron to lower the melting temperature of the semi-metallic conductor may conveniently be denominated a fusion method. In this method a conductor of any of the aforementioned compositions, preformed as aforedescribed, is pressed against the surface of an iron or iron alloy electrode, and the electrode is then heated, preferably inductively, until a very thin layer of the conductor becomes molten and fuses with the surface of the electrode. During such heating the iron migrates slowly into the adjacent surface of the conductor, reducing the melting point of a thin layer of the latter, as illustrated in Figure 2. Due to its thin section the molten layer rapidly approaches the compositions which solidify at temperatures below the phase transformation temperatures of the iron, as ndicated in Figure 2, to form the bond. Accordingly, the time of heating is only a matter of a few seconds, after which the assembly is allowed to cool.

In certain instances it is not feasible or convenient to preform the conductor 10 as aforedescribed, in which case it is possible to cast the conductor 10 and form the contact electrode simultaneously in accordance with the following method, for convenience denominated the direct casting method. Iron is placed in a mold, preferably of graphite, and the alloy of the conductor in chunk or granular form is also placed therein in contiguous engagement with the iron. The mold is then heated, preferably in a reducing atmosphere, to the melting point of the composition, that is, within the temperature range of 920 C. to 1100 C., for a short interval of time to produce limited alloying between the iron and the composition of the conductor. The mold. is then cooled causing the composition melt to solidify as an ingot firmly bonded to the iron electrode. The optimum temperature "17 {or contact formation in a hydrogen atmosphere has been found to lie in the aforementioned range, since above C. the alloying advances the rapidly to be accurately controlled, and below 920 C. it may be retarded by solid particles of the compositionwhich have not had 'time to absorb heat and melt.

The time of exposure at from at least 920 C. so 'at least 1100 C. must likewise be carefully controlled to prevent excessive alloying or solution or the iron of the electrode in the conductor and consequent impairment of the electrical properties thereof. The amount of iron which migrates into the melt depends upon the area of contact between the iron and the conductor composition and the volume of the latter, as well as the time of exposure. More specifically, the time of exposure at a given temperature within the aforeindicated range is proportional to the volume, and inversely proportional to the area of contact. Accordingly, we have found the maximum time of exposure in seconds at, for example 1100 C. (which will result in the migration of n o rnore than 0.5% of iron into the composition melt) can best be expressed as ranging from '12 to 45 times the ratio of the volume of the conductor composition to the area of engagement thereof with the electrode expressed in centimeters, depending upon whether the composition of the conductor 10 contains but a trace of tellurium or conversely is at the other end of the composition range aforedescribed wherein it contains but a trace of selenium.

For example, the time of exposure for a conductor of the aforedescribed lead-tcllurium of length 1.27 centimeters and diameter 0.635 centimeter, placed in a mold as aforeindicated with its end in contiguous engagement with an iron electrode as aforedescribed is less than 60 seconds. Under such conditions, the conductor can be cast on an alpha-stabilized iron electrode at 1100 C. for from 5 to 60 seconds without contaminating the conductor with more than 0.5% iron by precipitation thereof throughout the conductor as the mold is cooled.

In the formation of bonded contact electrodes by the direct casting method aforedescribed, it is necessary that the iron utilized be a phase-stabilized alloy of iron since the bond in this case is accomplished at a temperature C. or more, depending upon the composition of the conductor, above the transformation temperature of iron (this temperature being about 905 C. at which alphaphase iron (ferrite) transforms into gamma-phase (austenite)). Such phase stabilization is necessary to avoid shearing the solid bond between the conductor and the electrode during cooling.

It is, however, preferable that the iron forming the bonded electrode in the aforedescribed method be stabilized in the alpha phase because the iron migration rate is substantially lower in this case than in the case of gamma-phase-stabilized iron, and hence the control of the exposure time is less critical. For example, when the exposure time at 1100 C. is limited to 30 seconds for a sample of nominal size as aforeindicated, the iron content of the contacted conductor can be held below the limit of 0.5% by weight when the electrode is alphaphase stabilized.

Conventional alpha or gamma phase stabilizers well known to those skilled in the art may be utilized for the aforeindicated purposes. However, a preferred alpha stabilizer for high temperature contact is molybdenum, since the junction between the conductor and molybdenum-iron contact electrode appears to be more intirnate and freer of small blow-holes than most other alloys.

Thus, the preferred contact electrode for conductors of the aforcdescribcd compositions formed in accordance with the aforedescribed method is, we have discovered, alpha-stabilized iron and more particularly, iron stabilized in the alpha phase by the addition of from 2.7% to 7.0% molybdenum.

When a bonded electrode is formed bycither the direct to 0.25%, respectively,

substantially all tellurium,

follows: zirconium; 0.15%

'e'aliting'or fusion method aforcdescribed, it is preferable to anneal the contacted conductor subsequent to contact formation at from 540' C. M680 C. for from 20 to 10 hours to render the composition more homogeneous. It should also be understood that in the aforedes'cribed methods the iron should be substantially free of surface oxides and all contact formations accomplished under a reducing atmosphere since the composition of the conductor alloys poorly with iron if an oxide layer is present.

While'the direct casting method aforedescribed is somewhat simpler in that it permits casting of the conductor simultaneously with formation of the bonded iron contact electrode, the fusion method aforedes'c'ribed is advantageous in that it may be employed for alpha or gamma stabilized alloys as well as pure iron and unstabilized alloys. However, high carbon steel is not a desirable electrode in either method since high carbon concentration in iron lowers the iron transformation temperature below the melting points of the conductor plus iron composition shown in Figure 2. The fusion method has an added advantage in that the average iron concentration within the contacted conductor itself resulting from the bonding procedure is much less than in the direct casting method, and in fact, is, we have found, less than .01% by weight as an average proposition. Thus, even iron alloys containing relatively large concentrations of chromiurn, nickel, manganese, etc. (which are ordinarily detrimental to the electrical properties of the aforcdescribcd compositions of the conductor), can be used as contact electrodes without detrimental effect upon the electrical properties of the conductor due to their extremely small resulting concentration therein.

We claim:

1. An article of manufacture comprising an electrical conductor consisting essentially of lead and at least one member of the group consisting of selenium and tellurium, the lead content of the electrical conductor ranging froma minimum of 61.95% by weight and a maximum of 65.0% by weight when the balance is substantially all tcllurium to aminimum of 72.45% and a maximum of 75.0% by weight when the balance is substantially all selenium, and a contact electrode of iron in contiguous engagement therewith.

2. An article of manufacture comprising an electrical conductor consisting essentially of lead and tellurium, the leadcomprising 61.95% to 65.0% by weight of the conductor, the balance being substantially all tellurium, and a contact electrode of iron in contiguous association therewith.

3. An article of manufacture comprising an electrical conductor consisting essentially of lead and tellurlum, the lead comprising 61.95% to 63.0% by weight of the conductor, the balance being substantially all tellurium, the conductor containing no more than 0.001% by weight of deleterious impurity, and the conductor containing an effective amount of a third element promoter affording the conductor reproducible variation of and control over the electrical properties thereof, and a contact electrode of iron in contiguous association therewith.

4. An article of manufacture comprising an electrical conductor consisting essentially of lead and tellurium containing 61.95% to 63.0% lead by weight, and the balance and at least one member selectcd from the group consisting of, bismuth, tantalum, manganese, zirconium, titanium, aluminum, gallium, chlorine, bromine, iodine, uranium, sodium, potassium, thallium and arsenic in an amount not in excess by weight percent of the lead and tellurium of said members as 0.50% tantalum; 0.25% manganese; 0.25% titanium; 0.10% aluminum; 0.25% gallium; 0.10% chlorine; 0.20% bromine; 0.25% iodine; 0. uranium; 0.06% sodium; 0.10% potassium; and bismuth, thallium and arsenic not in excess of from 0.60% to 1.20%, and from 0.25% to 1.00% and from 0.07% over the aforementioned range 19 of lead, and a contact electrode of iron in contiguous association therewith.

5. An article of manufacture according to claim 1 in which the contact electrode is bonded to the electrical conductor.

6. An article of manufacture according to claim 1 in which the contact electrode is bonded by fusion to the electrical conductor.

7. An article of manufacture according to claim 1 in which the iron of the contact electrode is phase stabilized and is bonded by casting to the electrical conductor.

8. An article of manufacture according to claim 24 in which the contact electrode is bonded by fusion to said electrical conductor.

9. The method which comprises placing an electrical conductor consisting essentially of lead and at least one member of the group consisting of selenium and tellurium in contiguous engagement with a contact electrode of iron, applying heat to the latter sufficient to permit migration of the iron into the electrical conductor to form a layer of an alloy thereof having a melting point below the phase transformation temperature of the iron, and immediately fusing said layer and the electrode to provide a bonded conductor-electrode unit.

10. The method which comprises placing a composition consisting essentially of lead and at least one member of the group consisting of selenium and tellurium in contiguous engagement with a contact electrode of alphastabilized iron, and heating to a temperature of from at least 920 C. to at least 1100" C. to form a bonded therebetween.

11. An article of manufacture comprising an electrical conductor consisting essentially of lead and selenium, the lead comprising 72.45% to 75.0% by weight of the conductor, the balance being substantially all selenium, and a contact electrode of iron in contiguous association therewith.

12. An article of manufacture comprising an electrical conductor consisting essentially of lead and selenium, the lead comprising 72.45% to 73.50% by weight of the conductor, the balance being substantially all selenium, the conductor containing no more than 0.001% by weight of iron in contiguous association therewith.

13. An article of manufacture comprising a negative electrical conductor consisting essentially of lead and selenium containing 72.45% to 73.50% lead by weight, and the balance substantially all selenium, and containing by weight of other matter except for 1odine, chlorine, bromine, zirconium, silicon, titanium, tantalum, gallium, aluminum, copper, gold, bismuth, antimony, fluorine and columbium, and except for not more by weight than 002% sodium, 003% potassium and .002% lithium, and except for not more than from .04% to .08% thallium and from .02% to 06% arsenic, respectively, over the aforementioned range of lead, and a cgntact electrode of iron in contiguous association there- Wlt 14. An article of manufacture comprising an electrical conductor consisting essentially of lead and selenium containing 72.45% to lead by weight, the balance mine, zirconium, silicon, titanium, indium, lium, aluminum, copper, gold, bismuth, antimony, fluorine, columbium, sodium, thallium, potassium, lithium and arsenic in an amount not in excess by weight percent of the lead and selenium of said members as follows: 0.50% iodine; 0.20% chlorine; 0.60% bromine; 0.60% zirconium; 0.10% silicon; 0.10% titanium; 0.20% indium; 0.60% tantalum; 0.15% gallium; 0.03% aluminum; 0.30% copper; 0.35% gold; 0.02% fluorine; 0.35% columbium; 0.08% sodium; 0.15% potassium; 0.03% lithium; and bismuth, antimony, thallium and arsenic not in excess of from 0.40% to 2.5%, from 0.20% to 1.5%, from 0.72% to 1.5%, and from 0.10% to 0.30%, respectively, over the aforementioned range of lead, and a contact electrode of iron in contiguous association therewith.

15. An article of manufacture comprising a negative electrical conductor consisting essentially of lead and selenium containing 72.45% to 73.50% lead by Weight, the balance substantially all selenium, and containing not more than 0.001% by weight of other matter except for a member of the group consisting of iodine, chlorine, bromine, zirconium, silicon, titanium, indium, tantalum, gallium, aluminum, copper, gold, bismuth, antimony, fluorine and columbium affording reproducible variation of and control over the electrical properties of the conductor, and a contact electrode of iron in contiguous association therewith.

16. An article of manufacture comprising an electrical conductor consisting essentially of lead and selenium, the lead comprising 72.45% to 73.50% by weight of the conductor, the balance substantially all selenium, said conductor containing not more than 0.001 by weight of other matter except for a member of the group consisting of iodine, chlorine, bromine, zirconium, silicon, titanium, indium, tantalum, gallium, aluminum, copper, gold, bismuth, antimony, fluorine, columbium, sodium, thallium, potassium, lithium and arsenic for affording reproducible variation of and control over the electrical properties of the conductor, and a contact electrode of iron in contiguous association therewith.

17. An article of manufacture according to claim 16 in which the contact electrode is bonded by fusion to said electrical conductor.

18. An article of manufacture comprising an electrical conductor consisting essentially of lead and selenium, the lead comprising 72.45% to 73.50% by weight of the conductor, the balance substantially all selenium, said conductor containing not more than 0.001% by weight of other matter except for a member of the group consisting of iodine, chlorine, bromine, zirconium, silicon, titanium, indium, tantalum, gallium, aluminum, copper, gold, bismuth, antimony, fluorine, columbium, sodium, thallium, potassium, lithium and arsenic affording reproducible variation of and control over the electrical properties of the conductor, and a contact electrode of phase-stabilized iron bonded by casting to said electrical conductor.

19. An article of manufacture comprising an electrical conductor consisting essentially of lead and at least one member of the group consisting of selenium and tellurium, the conductor containing no more than 0.001% by weight deleterious impurity, and the conductor containing an effective amount of a third-element promotor affording the conductor reproducible variation of and control over the electrical properties thereof, and a contact electrode of iron in contiguous association therewith.

20. An article of manufacture comprising an electrical conductor consisting essentially of lead and at least one member of the group consisting of selenium and tellurium, the lead content of the electrical conductor ranging from a minimum of 61.95% by weight and a maximum of 63.0% by weight when the balance is substantially all tellurium, to a minimum of 72.45 and a maximum of 73.50% when the balance is substantially all selenium, said conductor further comprising a third element promoter selected from the following group when the balance of the conductor is substantially all tellurium: Bismuth. tantalum, manganese, zirconium, titanium, aluminum, gallium, chlorine, bromine, iodine, uranium, sodium, potassium, thallium and arsenic affording reproducible variation of and control over the electrical properties of the conductor; and selected from the following group when the balance of said conductor is substantially all selenium: Iodine, chlorine, bromine, zirconium, silicon, titanium.

indium, tantalum, gallium, aluminum, copper, gold, bismuth, antimony, fluorine, columbium, sodium, thallium, potassium, lithium and arsenic affording reproducible variation of and control over the electrical properties of the conductor; and a contact electrode of iron in contiguous association therewith.

21. An article of manufacture comprising a positive electrical conductor consisting essentially of lead and tellurium containing 61.95% to 63.0% lead by weight, the balance substantially all tellurium, and containing not more than 0.001% by weight of other matter, except for a member of the group consisting of sodium, potassium, thallium and arsenic, with the sodium and potassium in an amount by weight percent thereof to the lead and tellurium of not less than 0.0002% and 0.0004%, respectively, and thallium and arsenic in an amount not less than from 0.005% to 0.02% and from 0.0008% to 0.002%, respectively, over the aforementioned range of lead, and a contact electrode of iron in contiguous association therewith.

22. An article of manufacture comprising a negative electrical conductor consisting essentially of lead and tellurium containing 61.95% to 63.0% lead by weight, the balance substantially all tellurium, and containing not more than 0.001% by weight of other matter, except for a member of the group consisting of bismuth, tantalum, manganese, zirconium, titanium, aluminum, gallium, chlorine, bromine, iodine, uranium, sodium, potassium, thallium and arsenic, with the bismuth, tantalum, manganese, zirconium, titanium, aluminum, gallium, chlorine,

bromine, iodine and uranium in an effective amount afiording reproducible variation of and control over the electrical properties of the conductor; the sodium and potassium not more by weight than 0.0002% and 0.0004%, respectively, over the aforementioned range of lead; and the thallium and arsenic not more by weight than from 0.005% to 0.02% and 0.0008% to 0.002%, respectively, over the aforementioned range of lead; and a contact electrode of iron in contiguous association therewith.

23. An article of manufacture comprising a negative electrical conductor consisting essentially of lead and tellurium containing 61.95% to 63.0% lead by weight, balance substantially all tellurium, and containing not more than 0.001% by weight of other matter, except for an effective amount of a member of the group consisting of I bismuth, tantalum, manganese, zirconium, titanium, aluminum, gallium, chlorine, bromine, iodine and uranium for affording the reproducible variation of and control over the electrical properties of the conductor, and a contact electrode of iron in contiguous association therewith.

24. An article of manufacture comprising an electrical conductor consisting essentially of lead and tellurium, the lead comprising 61.95% to 63.0% by weight of the conductor, balance substantially all tellurium, and containing not more than 0.001% by weight of other matter, except for an effective amount of a member selected from the group consisting of bismuth, tantalum, manganese, zirconium, titanium, aluminum, gallium, chlorine, bromine, iodine, uranium, sodium, potassium, thallium and arsenic for affording the reproducible variation of and control over the electrical properties of the conductor, and a contact electrode of iron in contiguous association therewith.

25. An article of manufacture comprising an electrical conductor consisting essentially of lead and tellurium, the lead comprising 61.95% to 63.0% by weight of the conductor, balance substantially all tellurium, and containing not more than 0.001% by weight of other matter, except for an effective amount of a member selected from group consisting of bismuth, tantalum, manganese, zirconium, titanium, aluminum, gallium, chlorine, bromine, iodine, uranium, sodium, potassium, thallium and arsenic affording the reproducible variation of and control over the electrical properties of the conductor, and a contact electrode of phase-stabilized iron bonded by casting to said electrical conductor.

26. An article of manufacture comprising a positive electrical conductor consisting essentially of lead and selenium containing 72.45% to 73.50% lead by weight, balance substantially all selenium, and containing not more than 0.001% by weight of other matter, except for a member of the group consisting of sodium, potassium, lithium, thallium and arsenic, with the sodium, potassium and lithium in an amount by weight percent thereof to the lead and selenium of not less than 0.002%, 0.003% and 0.002%, respectively; and thallium and arsenic in an amount not less than from 0.04% to 0.08% and from 0.02% to 0.06%, respectively, over the aforementioned range of lead, and a contact electrode of iron in contiguous association therewith.

27. An article of manufacture comprising a negative electrical conductor consisting essentially of lead and selenium containing 72.45% to 73.50% lead by weight, balance substantially all selenium, and containing not more than 0.001% by weight of other matter, except for a member of the group consisting of iodine, chlorine, bromine, zirconium, silicon, titanium, indium, tantalum, gallium, aluminum, copper, gold, bismuth, antimony, fluorine, columbium, sodium, potassium, lithium, thallium and arsenic, with the iodine, chlorine, bromine, zirconium, silicon, titanium, indium, tantalum, gallium, aluminum, copper, gold, bismuth, antimony, fluorine, and columbium in an elfective amount affording reproducible variation of and control over the electrical properties of the conductor; the sodium, potassium and lithium not more by weight than 0.002%, 0.003% and 0.002%, respectively; and the thallium and arsenic not more by weight than from 0.04% to 0.08% and 0.2% to 0.06%, respectively, over the aforementioned range of lead; and a contact electrode of iron in contiguous association therewith.

References Cited in the file of this patent 1000, M. Hansen. 

1. AN ARTICLE OF MANUFACTURE COMPRISING AN ELECTRICAL CONDUCTOR CONSISTING ESSENTIALLY OF LEAD AND AT LEAST ONE MEMBER OF THE GROUP CONSISTING OF SELENIUM AND TELLURIM, THE LEAD CONTENT OF THE ELECTRICAL CONDUCTOR RANGING FROM A MINIMUM OF 61.95% BY WEIGHT AND A MAXIMUM OF 65.0% BY WEIGHT WHEN THE BALANCE IS SUBSTANTIALLY ALL TELLURIUM TO A MINIMUM OF 72.45% AND MAXIMIUM OF 75.0% BY WEIGHT WHEN THE BALANCE IS SUBSTANTIALLY ALL SELENIUM, AND A CONTACT ELECTROD OF IRON IN CONTIGUOUS ENGAGEMENT THEREWITH. 